Spectroscopic Requirements of CH4 for MERLIN - a Challenge

The planned space-borne differential LIDAR instrument MERLIN uses two spectral points in the 1.64 um wavelength region to derive weighted CH4 columns. The differential spectroscopic error, i.e. column change by spectroscopic error due to different atmospheric profiles or different ground altitudes below 2 km, should be below 0.623 ppb (0.033%). Within the German-funded SMERLIN project, retrieval simulations are carried out to specify the error requirements for individual line parameters and for systematic error sources in the laboratory measurement and analysis. Cavity ring-down spectroscopy (CRDS) measurements of CH4 carried out at NIST are utilized to derive requirements. The HITRAN spectroscopic database would be too far from reality to be used for this task, especially since important line parameters as speed dependence and line mixing are missing. Excellent laboratory measurements are advantageous for deriving requirements especially since realistic fit residuals are available. It will be shown that the non-noise fit residuals of the multispectrum fitting of the laboratory spectra, even when they are below the 0.1% level, are the largest error source and that broadening of CH4 by H2O must be considered. The current status of the spectroscopic database on CH4 will be reviewed and the requirements for future measurements shown. Furthermore, the modification needs for the level 2 processor to fulfill requirements based on the findings of the retrieval study will be discussed.